Incoming Solar Radiation

Solar Radiance arrives:

at the Top Of the Atmosphere (TOA) undepleted

Solar constant

Radiant energy received at the TOA per unit time, per unit surface area perpendicular to the Sun’s rays, called solar irradiance… often

Irradiance

units of Wm^-2 , irradiance also decreases with distance from the source and is defined as the power of solar radiation received per unit area.

Irradiance pattern

Solar energy travels undepleted through space BUT irradiance decreases with distance from the sun

When is Earth closest to the sun?

closest to the Sun in early January and furthest away in early July

How much does the earth to the sun distance vary? What is the mean distance?

about ±1.5%, mean Earth-sun distance D= 1.496 × 10 ¹¹ m

Is the radiant energy flux at the Earth surface expected to be less or more than the typical calue of S₀=1367Wm^-2?

less because of several different processes in the atmosphere that attenuate the solar radiation (absorption, reflection, scattering…)

Solar irradiance S₀ refers to:

the radiant flux received at the TOA per unit time, per unit surface area perpendicular to the Sun’s rays

For spherical Earth

The projection of the sunlit hemisphere on a plane at right angles to the sun rays (surface is perpendicular to the Sun’s rays)

Describe the rays that hits the Earth

sun is at a very large distance from Earth so the rays that hit the terrestrial disk are practically parallel to one another

Projection is?

a circular disk of radius equal to the Earth’s radius

At summer solstice (21 june)

Disk tangent to the Tropic of Cancer at the time of the summer solstice

At winter Solstice (21 december)

disk tangent to the tropic of Capricorn

Earth’s Radiative Equilibirum

to maintain temperature constant - there is no energy gain or loss due to radiation